Method for producing a diffusion barrier and polymeric article having a diffusion barrier

ABSTRACT

A method of forming a diffusion barrier on an article of a polymer blend of (i) a high surface energy polymer and (ii) a low surface energy polymer. Most commonly the low surface energy polymer is an organosilicon polymer, as a polysilane or a polysiloxane. The surface of the article is exposed to ozone and ultraviolet radiation to form a diffusion barrier.

FIELD OF THE INVENTION

This invention relates to diffusion barriers on polymeric articles, andto methods of preparing the diffusion barriers. According to theinvention disclosed herein, a polymer blend alloy is prepared containinga high surface energy component and a low surface energy component. Thearticle is then subjected to an ozone containing atmosphere in thepresence of ultra violet radiation to form the diffusion barrier. Thediffusion barrier is formed by the partial oxidation of the low surfaceenergy component that has diffused to the surface. Exemplary low surfaceenergy components are polysilanes, e.g., having --Si--Si--repeatingunits, exemplified by --Si(CH₃)₂ --! and the like, and polysiloxanes,e.g., having --Si--O--repeating units, exemplified by --Si(CH₃)₂ O--!and the like.

BACKGROUND OF THE INVENTION

For polymer blends, that is, physical mixtures of two or more polymersor copolymers that are not linked by covalent bonds, and that containone or more components having lower surface energy than the bulkpolymer, polymers, or copolymers, segregation of the low surface energycomponents to the surface can occur. This results in a hydrophobicsurface and inhibits the ability to transfer materials (e.g., inks,paints, dyes) to the surface of an article comprising the blend. Thissegregation also may result in poor interfacial adhesion between appliedlayers, films, coatings, adhesives, and the like, and underlyingarticles comprising the blend.

In other cases, for a homogeneous polymer system, inward diffusion ofmoisture or other chemicals/materials into the bulk may be a problem.This can result in degradation of the properties of the article.

Thus, there exists a need for surface modification of articlesfabricated of polymer blends or alloys to prevent segregation andhydrophobicity, and to enhance the wettability and bondability of thesurface.

SUMMARY OF THE INVENTION

For polymer blends, the chemical transformation of the segregatedmaterial into a diffusion barrier has been achieved according to ourinvention. This transformation retards further segregation of the lowsurface energy component(s) to the surface.

In addition, the modified surface can act as a barrier to inwarddiffusion of moisture or other undesirable materials.

For a single polymer system, doping of the polymer with a component orcomponents having lower surface energy than the bulk, followed byoxidation with ozone in the presence of ultra violet (uv) radiation willresult in a diffusion barrier and a more stable surface with respect toreactions with the environment.

The polymeric body is treated with reactive oxygen (ozone) and UVradiation. The apparatus for this technique is quite modest, usuallyconsisting of a UV source, e.g., a low-pressure mercury vapor lamp, anda chamber to house the UV source and the material being treated. Theozone is the photolysis product of oxygen in the presence of a source,as a mercury vapor light source, emitting 184.9 nanometer radiation.Treatment is almost always performed in air at atmospheric pressure toget ozone.

The method of this invention is particularly useful withorganosilicon/organic polymer blends. By organosilicons are meantpolysilanes and polysiloxanes, where polysilanes are polymers having--Si--Si--repeating units, exemplified by --Si(CH₃)₂ --! and the like,and polysiloxanes are polymers having --Si--O-- repeating units,exemplified by --Si(CH₃)₂ O--! and the like. In these systems conversionof organosilicon materials to silicon oxides is a phenomenon that iswell documented for exposure to oxygen plasma environments. Oxygenreactive ion etching of silicon-containing polymers results in aninitial thickness loss and a gradual slowing of polymer erosion untiletching ceases. During etching, it is believed that mobilesilicon-containing monomer or polymer diffuses to the polymer surfacewhere it is converted to SiO₂ or a suboxide thereof, and functions as anincreasingly effective etching barrier.

According to our invention, UV/ozone treatment of organic polymershaving organosilicon additives results in formation of a thin,protective barrier that inhibits diffusion of bulk material to thesurface, inhibits diffusion of material from the environment into thebulk, and inhibits environmental contamination of the surface.

This is achieved by doping the bulk polymer or a surface portion thereofwith a suitable organosilicon polymer or monomer additive at anappropriate concentration. The doped blend is then subjected to exposurein a UV/ozone environment such that a thin, stable, protective barrieris formed at the surface.

The method of this invention, and the resulting products areparticularly useful, for example,

(1) to enhance printability and adhesion of inks to organosiliconcontaining polymers, for example, encapsulated products withorganosilicon-containing encapsulants (e.g., Dexter Hysol 4450);

(2) to enhance the moisture and chemical resistance of polymers, such asencapsulants, in general, by doping with a suitable additive andtreating in UV/ozone;

(3) to enhance the moisture and chemical resistance of other polymersused for a variety of applications requiring diffusion barriers and/orstable surface properties.

A further advantage is that unlike barriers produced by depositionprocesses, the method of the invention is self-patterning, i.e., thematerial that constitutes the protective barrier is formed only on theorganosilicon-bearing material, not deposited on other areas of thesubstrate. Also, since the barrier formed by this method is, by thenature of the technique, incorporated as part of theorganosilicon-bearing material, not as a separate, deposited layer,adhesion of the barrier to the bulk is high. In addition, the barrier,which is silicon dioxide or a suboxide thereof, is optically transparent(in the visible, and into the 185 nanometer ultraviolet and 1140nanometer infrared bands) and hydrophilic. The method of the inventionis also less expensive than conventional means of producing barrierfilms.

THE FIGURES

The invention may be understood by reference to the FIGURES appendedhereto.

FIG. 1 shows the water contact angle on a film of a organosiliconpolymer--polyepoxide polymer blend as a function of surface treatmentand storage times.

FIG. 2 shows the x-ray photoelectron spectroscopy pattern of a sample ofa organosilicon polymer--polyepoxide polymer blend after UV/ozonesurface treatment.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention described herein, for polymer blends, thechemical transformation of the segregated material into a diffusionbarrier has been achieved. The method of this invention utilizes apolymer blend or polymer of a high surface energy material, as anorganic polymer, and an excess of a low surface energy material, such asa organosilicon containing polymer, exemplified by a silane, such as analkyl silane, or a siloxane, having the formula R--Si(OCH₃)₃,R--Si(OCH₃)₂ CH₃, or R--Si(OCH₃)(CH₃)₃, where R is an alkyl or vinylgroup. By "high" and "low" surface energy materials are meant misciblematerials differeing in critical surface tension, for example by 5dynes/centimeter and preferable 10 dynes/centimeter or more. The lowsurface energy material diffuses toward the surface, where it isconverted to an oxide, e.g., SiO_(x), where x is between 1.6 and 2.0.This transformation or oxidation retards further segregation of the lowsurface energy component(s) to the surface, and provides a modifiedsurface.

In addition, the modified surface can act as a barrier to inwarddiffusion of moisture or other undesirable materials.

Unlike barriers produced by deposition processes, the method of thisinvention is self-patterning, i.e., the material that constitutes theprotective barrier is formed only on the organosilicon-bearing material,not deposited on other areas of the substrate. Also, since the barrierformed by this method is, by the nature of the technique, incorporatedas part of the organosilicon-bearing material, not as a separate,deposited layer, adhesion of the barrier to the bulk is very high. Inaddition, the barrier is optically transparent (in the visible, and intothe 185 nanometer ultraviolet and 1140 nanometer infrared bands) andhydrophilic. This method is also less expensive than conventional meansof producing barrier films.

For a single polymer system, doping of the bulk polymer with a componentor components having lower surface energy than the bulk, followed byoxidation with ozone in the presence of ultra violet radiation, willresult in a diffusion barrier and a more stable surface with respect toreactions with the environment.

The method of this invention utilizes reactive oxygen and UV radiation.The apparatus for this technique is quite modest, usually consisting ofa UV source, e.g., a low-pressure mercury vapor lamp, and a chamber tohouse the UV source and the articles being treated. The ozone comes fromthe photolysis of oxygen. A low pressure mercury vapor source emitsradiation at wavelengths of 184.9 and 253.7 nanometers. Oxygenmolecules, O₂, absorb 184.9 nanometer radiation and dissociate to formatomic oxygen, O. The atomic oxygen, O, reacts with molecular oxygen,O₂, to form ozone, O₃. Thus, treatment is almost always performed in airat atmospheric pressure with the mercury vaport light source, althoughother ozone sources may be utilized. In comparison with plasma systems,UV/ozone surface treatment equipment is relatively inexpensive.

The method of the invention is to be distinguished from oxygen reactiveion etching. Conversion of organo-organosilicon materials, as silanesand siloxanes, to silicon oxides is a phenomenon that is well documentedfor exposure to oxygen plasma environments, i.e., oxygen reactive ionetching. Oxygen reactive ion etching of silicon-containing polymersresults in an initial thickness loss and a gradual slowing of polymererosion until etching ceases. During etching, it is believed thatsilicon-containing monomer diffuses to the polymer surface where it isconverted to SiO₂ and functions as an increasingly effective etchingbarrier.

The method of this invention and the articles produced thereby can beprepared from various polymer blends. The preferred polymer blends arecharacterized by miscibility of the constituents, and an excess of thelow surface energy component, where the "high" and "low" surface energyconstituents differ in surface tension, preferably by 5 to 10dynes/centimeter or more. Exemplary silanes include those generallycommercially available silanes, such as dimethyl silane, and exemplarysiloxanes are those having the formula R--Si(OCH₃)₃, R--Si(OCH₃)₂ CH₃,or R--Si(OCH₃)(CH₃)₃, where R is an alkyl or vinyl group, and includemethyl, ethyl, propyl, and vinyl siloxanes. Exemplary organic polymersinclude polyvinyls, polyepoxides, polycarbonates, polyimides, andpolyurethanes. Generally, the preferred polysilanes and polysiloxaneshave a surface tension below about 25 to 30 dynes per centimeter, andthe preferred organic polymers have a surface tension above about 25 to30 dynes per centimeter. The preferred blends are within the range ofmiscibility of the constituents and contain an excess of the low surfaceenergy organosilicon constituent. Especially preferred are polymerblends of poylsiloxanes and polyepoxides.

The invention is illustrated by the following example.

EXAMPLE

Samples of a commercially available epoxy-based encapsulant (DexterHysol 4450) containing some organosilicon and other inorganic fillerswere exposed to UV/ozone, oxygen plasma, and flame treatments forvarious durations.

As shown in FIG. 1, advancing DI water contact angle on the Dexter Hysolsurfaces were reduced from initial average values greater than 100degrees to a values less than 10 degrees for UV/ozone and plasmatreatments, and to less than 30 degrees for the flame treatment. Contactangles were then monitored as a function of aging time during storage inlab ambient conditions. As the figure shows, the UV/ozone treatedsurface maintains its high degree of hydrophilic character upon aging,while the plasma and flame-treated surfaces revert back to a morehydrophobic character. High resolution x-ray photoelectron spectroscopy(XPS) in the Si 2p photoemission band suggest that during UV/ozone andplasma treatments of the Dexter Hysol material, O--Si--C bonds in thesiloxane, observed prior to treatment, are converted to SiO_(x), where xis between 1.6 and 2.

This is illustrated in FIG. 2 which shows high resolution XPS spectra inthe Si 2p photoemission band for organosilicon samples before treatment(a), after two minutes of O₂ plasma treatment (b), and after 50 minutesof UV/ozone exposure (c). The spectrum of the untreated sample (a)contains contributions from both the silicon containing polymer (lowbinding energy) and glass filler (higher binding energy). Aftertreatment (b and c), the organosilicon is transformed into a glassysurface (higher binding energy). For this reason, it is believed thatthe signal in (b) and (c) is the result of transformation and notexposure of underlying glass-filler particles.

XPS examination of surfaces aged for greater than 40 days revealed thatthe UV/ozone-treated and plasma-treated surfaces retained a strongSiO_(x) contribution, while the flame-treated surfaces at no timeexhibited the SiO_(x) character (i.e., at all times maintained aO--Si--C character).

The increase in contact angle following treatment for plasma-treated andflame-treated parts is due primarily to a combination of two phenomena;(1) some diffusion of organosilicon material from the bulk of theencapsulant to the surface, and (2) changes in surface groups, e.g.,decrease in carbon-oxygen groups. Both of these factors influence thesurface wetting properties. The apparent lack of reversion in contactangle of the UV/ozone-treated encapsulant surface to its original stateis indicative of the formation of a thin, more stable protective barrieragainst diffusion. In addition, the observation that theUV/ozone-treated surface maintains a highly hydrophilic nature indicatesthat in addition to resisting diffusion, the UV/ozone-treated surface isresistant to contamination from the environment.

Although the exact mechanisms leading to these differences in thesurface properties resulting from each of the treatments are unknown atthis time, it is believed that the presence of the intense UV exposureand/or the absence of bombardment by kinetically energetic particles inthe UV/ozone system may impart such favorable properties. In addition,since material removal is more pronounced in the plasma system, theformation of the protective barrier may be less effective than thatproduced using UV/ozone treatment.

UV/ozone treatment of organic polymers having low surface energyorganosilicon additives results in formation of a thin, protectivebarrier that inhibits diffusion of bulk material to the surface,inhibits diffusion of material from the environment into the bulk, andinhibits environmental contamination of the surface.

This is achieved by doping the polymer with a suitable organosiliconadditive at an appropriate concentration. The doped blend is thensubjected to exposure in a UV/ozone environment such that a thin,stable, protective barrier is formed at the surface.

Several ink formulations were tested for adhesion (Tape Tests) onsurfaces of the Dexter Hysol 4450 polysiloxane-polyepoxide polymer blendencapsulant treated using a variety of techniques. Ink was applied tothe surface of the surface treated samples, and then the ink was testedfor adhesion by consumer adhesive tape. Results are given in the tablesbelow.

                  TABLE 1                                                         ______________________________________                                        Results of Example 1 "As Formed" tape tests.                                  INK           PLASMA    UV-OZONE   CONTROL                                    ______________________________________                                        DEXTER        PASS      PASS       PASS                                       DEXTER W/ IPA NA        PASS       PASS                                       TRA. TECH M2  PASS      NA         NA                                         TRA. TECH B/GL                                                                              PASS      PASS       PASS                                       MARKEM 4481   PASS      PASS       FAIL                                       AIS           PASS      PASS       FAIL                                       ______________________________________                                         NOTE: FAILS ALWAYS OCCURRED ON BOTH SCOTCH AND KAPTON TAPES AND ON LINES      GOING BOTH INLINE AND PERPENDICULAR TO THE TAPE LIFT DIRECTION.          

The tests above were repeated after five weeks of aging at ambientconditions of approximately 20 degrees C. and 30-70 percent relativehumidity,, and the following results were obtained.

                  TABLE II                                                        ______________________________________                                        Results of Parts in Table I retested after five weeks of                      aging.                                                                        INK           PLASMA    UV-OZONE   CONTROL                                    ______________________________________                                        DEXTER        PASS      PASS       PASS                                       DEXTER W/ IPA NA        PASS       PASS                                       TRA. TECH M2  PASS      NA         NA                                         TRA. TECH B/GL                                                                              PASS      PASS       PASS                                       MARKEM 4481   PASS      PASS       FAIL                                       AIS           PASS      PASS       PASS                                       ______________________________________                                    

Comparison with the wettability tests shown in FIG. 1 showed that theUV-ozone treated samples were more stable then the plasma and controlsamples.

                  TABLE III                                                       ______________________________________                                        Results of tape test matrix.                                                  INK     PLASMA   UVOZONE   FLAME OZONE CONTROL                                ______________________________________                                        DEXTER  PASS     PASS      FAIL  FAIL  PASS                                   T.T. B/GL                                                                             PASS     PASS      PASS  FAIL  PASS                                   MARKEM  PASS     PASS      FAIL  FAIL  FAIL                                   4481                                                                          AIS     PASS     PASS      PASS  FAIL  FAIL                                   ______________________________________                                    

The results shown in Tables I and III are consistent. Furthermore,treatment with flame and ozone-only (no UV exposure) do not improvemarkability. In fact, in some instances, adhesion becomes worse withthese treatments. However, the plasma and uv/ozone treatmentsconsistently show positive results independent of ink type. Also, TableII illustrates that no degradation over time occurs for markability onparts treated using plasma or uv/ozone processes. Table III also showsgood results independent of ink chemistry.

Although both UV/ozone and plasma treatments result in improvedmarkability over extended periods of time (i.e., long shelf life withrespect to markability), the UV/ozone treatment results in a more stablesurface as inferred from contact angle measurements with respect tohydrophilic properties. In this regard see FIG. 1.

While the invention has been described with respect to certain preferredembodiments and exemplifications, it is not intended to limit the scopeof the invention thereby, but solely by the claims appended hereto.

We claim:
 1. A method of forming a diffusion barrier on a surface of an article of a polymer blend of two polymers different in surface tension wherein the polymer having a lower surface energy than the other is capable of undergoing partial oxidation when exposing to ozone and ultraviolet radiation comprising exposing the article to ozone and ultraviolet radiation to partially oxidize the lower surface energy polymer that has diffused to the surface of the article and to form a diffusion barrier thereon.
 2. The method of claim 1 wherein polymers differs in surface tension of at least 5 dynes/centimeter.
 3. The method of claim 2 wherein the diffusion barrier is a layer of SiO_(x) where x is between 1.6 and 2.0.
 4. The method of claim 1 wherein polymers differs in surface tension of at least 10 dynes/centimeter.
 5. The method of claim 1 wherein the polymer having a lower surface energy is chosen from the group consisting of silanes and siloxanes.
 6. The method of claim 5 wherein the polymer having a lower surface energy is a siloxane.
 7. The method of claim 1 wherein the diffusion barrier is transparent at least from the 1140 nanometer infrared band through the visible band to the 185 nanometer ultraviolet band.
 8. The method of claim 1 wherein the polymer having a higher surface energy is chosen from the group consisting of polyalkyls, polyvinyls, polyepoxides, polycarbonates, polyimides and polyurethanes.
 9. The method of claim 1 wherein the polymer having a lower surface energy is present as a dopant near the surface of the article.
 10. The method of claim 1 wherein the polymer having a lower surface energy is dispersed throughout the article.
 11. A method of forming a diffusion barrier on a surface of an article of a blend of organosilicon and organic polymers different in surface tension wherein the organosilicon polymer having a lower surface energy than the other is capable of undergoing partial oxidation when exposing to ozone and ultraviolet radiation comprising exposing the article to ozone and ultraviolet radiation to partially oxidize the organosilicon polymer that has diffused to the surface of the article and to form an SiO_(x) diffusion barrier thereon, where x is between 1.6 and 2.0. 